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Beyond Streamlining: Drag reduction in the 21st century

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John McGinnis
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#1 Posted: 11/19/2009 02:40:44 Modified: 12/1/2009 13:22:22


fuselage flow.jpg

 

This post is nerd candy; a lure for attracting some of our lunker aerodynamicists to the surface. Among the shocking but true insights to be discussed is the one most responsible for a half century of stagnation in drag reduction:

It takes HALF the power to fly at a given speed if the energy is used differently.

Early on it was decided that since the thrust required for level flight equals the drag, obviously the power needed should be that required to make that much thrust.

Not true.

We fly by moving air; usually, far more of it than necessary. If power is used partially to overcome drag and partially to make thrust, we double our efficiency. Less power= less weight=less fuel=less structure=less weight=less fuel=less structure...

Everyone likes to talk about how hard, complicated, and expensive this is. They base this conclusion on old tests in which it was complicated and expensive to apply to jets at 600MPH.

In fact, it's quite simple and easy. A little disruptive, I'd agree. But we spend far more time and effort on things that won't make one percent as much difference.

Let's talk here about some of the proven drag reduction technologies that we're not using. Since the object is to apply these things to new solutions, rather than continue to fail in their application to old airframes, please pretend that we are back in the 30's, 40's. and 50's, and that anything is possible.

Because it still is.

 
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Jim Heffelfinger
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#2 Posted: 11/19/2009 14:40:29

Great topic - you have my ears.......



John McGinnis
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#3 Posted: 11/19/2009 16:35:27 Modified: 11/27/2009 21:49:32

Starter Topics:

  

Natural Laminar Flow


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Span Efficiency             
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Reynolds Number
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Wake Immersed Propulsion
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 Non-Planar Configuration


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Open Thermodynamics        engine wire.jpg

 

Active Boundary Layer Control

Pressure Thrust

Wave Drag

 
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Tom DeMarino
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#4 Posted: 11/19/2009 18:53:59

Okay, with close to 150 emails a day, I usually gloss over this stuff, but this is near and dear to my nerdy little heart.  Having hung around Rutan's guys, worked with Paul McCready, and spent some time helping to set world altitude records, I have been exposed to some smart thinkers. Not to say that any rubbed off, but it does peak my interest. Let's hear how it goes. -TOm



Tom
Eric Peterson
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#5 Posted: 11/19/2009 19:14:18

John,

      I've thought of things like wings that are completely one piece molded(LWR+UPPER Skins to Spar)made out of some super light strong material that wouldn't need painting because you could mold it in any color that is weather and ultraviolet proof stuff. No fasteners. The control surfaces would be blended in with no gaps.

    I've read somewhere of synthetic rubber that moves with an electrical charge. That could be used to actuate the control surface. The rubber material would be flush where the gaps would normally be.

    Winglets reduce drag, add thrust and increase lift. Add those to the wing as well.

    I suppose this would be expensive in building and casting molds, material to boot, but its nerd candy.                                                                                                              Eric                                                                                                   

   



John McGinnis
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#6 Posted: 11/19/2009 21:51:05 Modified: 11/27/2009 21:53:48

Tom,

Thanks for taking the time. I was heavily influenced by MacCready and Rutan at an age when everything seemed easy and obvious. It's time to press on in their path for the benefit of the next generation. I'm happy to meet you here and I look forward to your input.

 
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Tom DeMarino
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#7 Posted: 11/19/2009 23:47:04

Well, John, I am thinking one cannot defy the laws of physics, but the answer may well lie there. I cannot remember the name of the gent that wrote the book on gaining speed and economy, but he was thinking out of the box a bit, and chronicled a number of solutions. Most of them, when looked at from an energy conservation point of view, make good sense.

I usually remind myself that the power required goes up as a function of the weight to the three halves power. First place to look for more energy is weight reduction.

Second place to look is where energy is used. The cooling drag is a big one. And there is energy in the exhaust, for instance. So using it to advantage can provide us some useful energy. Whether we use it to extract air from the cowl by augmenter stacks, increasing our cooling efficiency, or using it for thrust, which the stack will also accomplish, why waste it.

Of course, adding taper to a wing gets it closer to the ideal elliptical air foil, and reduces the structural weight significantly in the process. It can also give us storage near the fuselage for fuel, guns, etc. and it keeps the moment of inertia down to increase roll rate.

Another area where there is energy going to waste is the wing tip vortex. Using a Horner type tip realigns the flow and produces forward thrust. And you get the equivalent of increased span without making the parking area bigger (hence the appearance of them on airlliners) and without the bending moment of an increased span, keeping structural weight down

A similar method that Rutan borrowed from another aerodynamicist, and I apologize for not remembering his name or being a history buff, but he used the wing tip vortex when it rises again to provide up lift, which allows the efficiencies of a canard but with a rearward aerodynamic device. This puts the c.g. aft of where it would be with a conventional tail and allows all the aerodynamic devices to be  producing lift. This is hard to visualize, but think of a boom coming off the wing tip and projecting aft with a small horizontal and vertical tail behind the wing tip. A structural nightmare even on a flying wing, but it works to recapture the vortex and make use of it.

Another area to save is get rid of tails altogether, or make their necessity minimized. Make a flying wing or at least borrow from the concept. In order to be stable the aircraft needs a down force on the tail. One reason is the pitching moment of the airfoil. By choosing an air foil with a minimal pitching moment one can minimize the drag of the tail to balance it.

Those are my first thoughts on the subject, assuming I am getting close to the concept you had in mind.

 



Tom
John McGinnis
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#8 Posted: 11/20/2009 01:14:08 Modified: 11/27/2009 22:28:28

Eric,

 I happen to build snowboards using the technique you describe, with a printed topsheet material providing finish, graphics, and water/UV protection. It's a real possibility we'll soon see planes with such skins. Some mold-built wings are, functionally, at least, already there with various coatings and scrims in-mold. In all cases, keeping the weight down is a deal-breaker needing improvement.
office boards closeup.JPG

 I'm glad you place a priority on the control surface gap. It's not just the gap discontinuity causing flow disruption; the real drag source is the availability of a route for pressure equalization to be occurring between high and low pressure areas below and above the wing. This flow widens and turbulates the boundary layer, invisibly increasing the 'apparent' aft wing thickness.

Sealing the gap adds complexity and expense while introducing a liability into the system. Ideally in the planes of the future we simplify and reduce parts count, so your high tech 'synthetic muscle' actuator may have a home some day. For now, we can imagine that a controllable wing without a gap is better than one with, especially if it has simplified and reduced weight while maintaining present safety and reliability.

You wrote that winglets reduce drag, add thrust, and increase lift. Well, not necessarily. Now seems as good a time as any to introduce another important but wild concept that happens to be true. I'll come back to winglets shortly.

 

First thing we learn in aerodynamics is that there are four fundamental forces of flight: Thrust, lift, drag, and gravity. In steady level flight, thrust = drag and lift = weight. This statement is almost completely responsible for a frame of mind and a frame of reference that guarantees a poor result when dealing with viscous fluids. We may not fully understand why for quite a long time, but it will become perfectly clear in the future.

 There are only TWO fundamental forces of flight: Gravity, and viscous fluid inertia. Because low viscosity fluids swirl when pushed upon, drag, lift, and thrust become interchangeable and quite inseparable, even when we design for one or the other. We use four representative vectors for easy bookkeeping, and I'm not arguing to toss the math (yet), but the fact is that most aircraft are designed as if lift was lift, and drag was drag. It isn't for long.

As the aircraft drags previously stationary air along with it, the air swirls. As the plane pushes an air mass downward, which it must do in order to fly, air swirls in to take its place. Responding insightfully to these behaviors can create lift with surfaces that are not wings, and reduce drag with surfaces that add greatly to the aircraft wetted area. Future planes even create LIFT by pushing the aircraft down!

I'll be posting a brief intro to the implications of the Gabrielli-von Karman limit shortly. What it shows is that we've only been effective in designing efficient aircraft with long wingspan (gliders under 100 mph) and with large commercial jets (over 400 mph).

The reason we have failed to reach achievable high L/D at useful speeds under 400 mph is due to ignoring the critical influence of low viscosity. Going fast enough renders the air a near-solid, going slowly allows weaker structures of high aspect ratio.

Winglets opened the modern exploration of non-planar wing design, a technology acting in harmony with viscosity in three or four dimensions. (Biplanes and various flavors of polyhedral did so long ago.) Winglets can provide a high span efficiency by acting against the high velocity swirl at wingtips. Optimizations reveal that winglets are most effective after a span limit has been reached. Yet while span extension will deliver the most drag reduction, it is at the expense of structure suitable for higher speeds. Creating thrust with a winglet is generally sub-optimal.

A tall winglet, located to also provide lateral directional stability, provides very high span efficiency, which is one of the major advantages to Rutan derivative canards

 

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Unfortunately, the canard configuration itself reduces this advantage. The promise of non-planar concepts one or two steps beyond the winglet have yet to be fully explored, but a promising non-box-wing technology is now patent-pending, with particular application to efficient flight in the 200-400 mph regime.


 

 



John McGinnis
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#9 Posted: 11/20/2009 01:43:05

Tom,

Great stuff! I'm with you 100%, especially on not wasting heat energy that can produce thrust.

 By the way, we can't defy the laws of physics, but we certainly can ignore them well! Almost every equation used to design and evaluate an aircraft ignores or fails to accurately model the physics of viscous flow. They are representative simplifications, and it's a critical point.

The air seems to laugh at us as it calculates everything accurately in realtime. Think about it...

Some of our rules of thumb are about to become very dated as a result. Elliptical span loading being the 'ideal' began to be seriously questioned in the 90's, and it's simply not true for nonplanar configurations, such as wings having winglets. The assumptions the 'rules' depend upon should have alerted us to this some time ago.

Another assumption that 'makes sense' is that we should be trying always to create lift, but that's not always the case either.

What I have in mind is a comprehensive application of everything we know:

Stability is good!

Drag is bad!

Light is better!

You get the point. Everything we've seen that goes very far down these roads gets complicated and expensive. That will not work. Simple is the answer.

 

 



Joanne Palmer
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#10 Posted: 11/20/2009 11:43:15

Eric Peterson said

Winglets reduce drag, add thrust and increase lift. Add those to the wing as well.

I don't know of many aerodynamicists that would agree with that statement. 

Winglets do not, in themselves reduce drag any better than the similar increase in span.  If you increase the span you actually get a better reduction in drag and an increase in lift.  Winglets are used on production aircraft to gain the lift AND still remain within the aircraft space allocation. 

There is NO WAY that a passive device like a winglet can increase thrust. 

 

While this is a nice mental exercise, airplanes are fundamentally compromises and are built for all phases of flight.  A real fast low drag cruiser is a bear to slow down for landing.  Engines (and their cooling requirements) are sized for takeoff and climb.  The wheels are sized for the heaviest landing....etc.  Wings, by their nature, have to be compromises.  As do most things in life. 

 

 



John McGinnis
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#11 Posted: 11/20/2009 15:33:07

Joanne,

Thanks for your comments. This observation well illustrates my point concerning there being no practical distinction between lift, thrust, and drag. Eric's statement is certainly capable of being true, yet you are right in noting that winglets are not themselves a panacea. We only get about 45% of their height as apparent span increase, so whenever practical, a span increase will provide the greatest L/D increase for a fixed structural weight.

One must be careful, however, to note that the conclusion we reach for a winglet gets oversimplified into yes/no rules that impact non-planar designs in general. Such downstream prejudice is undesirable, since we may be failing to separate aerodynamic cost/benefit from structural cost/benefit in future application.

Several studies have suggested that winglets are an intermediary form, aerodynamically speaking. As we get more aero benefit, we start to see major penalties from a structural perspective. That factor is at the heart of matters, not their proven influence on induced drag reduction. A future solution will gather the benefits without the same structural penalties.

 

The 'thrust' associated with a winglet is not unlike the thrust associated with a glider. Indeed, it is the same phenomena, wrapped in poor terminology. Taking a glider from 24:1 to 40:1 by means of induced drag reduction is analagous to 'thrust', but it is really just revectoring of mass flow. A winglet can indeed be oriented and shaped to increase a thrust component, yet that will not cause maximum overall performance. As Kroo notes below, the optimum for a given bending moment constraint is to exert a wake influence similar to that provided by a span increase of 45% winglet height.

 

"The result that the sidewash on the winglet (in the Trefftz plane) is zero for minimum induced drag means that the self-induced drag of the winglet just cancels the winglet thrust associated with wing sidewash. Optimally-loaded winglets thus reduce induced drag by lowering the average downwash on the wing, not by providing a thrust component."

http://www.desktopaero.com/appliedaero/wingdesign/nonplanar.html

 

Regarding fundamental compromise, there can be none without a decision that accepts the undesirable aspect. We must therefore STOP accepting things like the idea that fast planes can't go slow and vice versa. There are trades to be made, but the premise I'm advancing here is that a lot of what we accept as true is only true because we haven't changed it in 50 years. Stay tuned to this thread and you'll probably see many of the solutions reach the light of day.



John McGinnis
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#12 Posted: 11/20/2009 23:41:07 Modified: 11/27/2009 22:37:45

The problem of truly efficient powered flight, in the speed range occupied by private and regional aircraft, is so profound as to have advanced no commercially viable solution in the entire history of man's effort. One factor underlying this failure is that we simply have not embraced the achievable limits.

By studying the ‘specific resistance’ of various forms of transportation, Gabrielli and von Karman discovered the boundary for the achievable L/D ratio at a given airspeed; known widely as the Gabrielli- von Karman limit (GvK). ("What Price Speed?" Mechanical Engineering, Vol. 72, October, 1950) Their primary chart, shown, reveals the conspicuous lack of efficient conveyance between 70 and 400 miles per hour. Bounded by autorail and airship at the low end, and by highly efficient large jet aircraft at high speeds, the transportation void between automobile and airliner is as notable for its lack of fuel efficiency as for its standard bearer: the fifty year old general aviation aircraft design.
Synergy GvK.jpg
Experience and subsequent updates of the GvK work have revealed that while technology improvements can be expected to push the achievable limits slightly toward greater efficiency over time, the fundamental relationship holds. By oversimplifying the mechanics of viscous fluid flows, then teaching the representative math of flight rather than the true physics, aeronautics effectively ‘fenced off’ a vast design space that few today even know exists. We might have seen airplanes capable of revealing this error before now, had a different development path been taken in the 1930s. As it stands, the GvK efficiency void literally maps out a colossal oversight in our science.


Special purpose aircraft designs, such as Burt Rutan's Voyager and Global Flyer designs used to circumnavigate the globe unrefueled, seem the only intruders into an otherwise unpopulated yet large design space above existing aircraft and below the GvK limit. Their performance well illustrates the gap between what is possible and what is accepted.

 The efficiency of an aircraft can be stated in terms of its lift to drag ratio, or L/D. All aircraft operate over a range of L/D based upon apparent fluid viscosity of the air and their flight configuration.

The specific resistance of a powered aircraft is its L/D ratio under full power at maximum weight. Goldschmied gave this handy number a name and gathered up the performance specifications for dozens of popular aircraft. His resulting Aerodynamic Efficiency Index (AEI) calculation allows all powered and unpowered aircraft to be compared meaningfully. Plotting the AEI of general aviation airplanes (see chart) against the GvK theoretical limit reveals the same vast opportunity for improvement noted previously.
AEI GA.jpg
Modern high performance composite sailplanes demonstrate an L/D as high as 60 to achieve more than 60% of the theoretical limit available at that (slow) speed. However, typical powered aircraft, which fly faster, seldom achieve L/D greater than 16, and worse yet, this figure tends to represent less than 25% of the GvK limit for the airspeed. Powered aircraft can therefore be said to be significantly LESS efficient as a result of the application of energy.
AEI void.jpg

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This is an unwelcome irony and one of the fundamental problems Synergy appears to solve. A source of energy should result in greater fluid dynamic efficiency.

The common thread among highly efficient aircraft prior to now is that they require long wingspans of high aspect ratio. Powered aircraft  that are fast enough also achieve high efficiency, yet long skinny wings quickly reach their strength limits when asked to carry more weight and go faster. What is needed is high span efficiency in the intermediate speed range: strong, fast wings of low induced drag.

nimbus 4.jpg
High powered aircraft that cannot achieve a high top speed are the most severely penalized designs. Ironically, this is the condition afflicting many market- appealing airplanes. As the chart shows, we have not ventured into the realm of efficient low-viscosity powered flight very often.

 

 



Eric Peterson
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#13 Posted: 11/25/2009 08:56:40 Modified: 11/25/2009 11:52:02

I heard from an aircraft engineer(from Gates Learjet) that winglets also act as a sail(although a sub-optimal one) getting a push or thrust. It made sense at the time.

I think its time for some new alien technology. Eric

 

 

 

 

 

 

 

 

 

 

 



John McGinnis
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#14 Posted: 11/25/2009 16:40:51 Modified: 1/11/2011 14:12:01

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Leigh Scott
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#15 Posted: 11/25/2009 22:29:53

Hi John,

"A little Knowledge is a dangerous thing!"

I am at the investigation stage of designing a major modification to install an LS1 Engine and PSRU in my Stinson 108-2 to replace the orphan Franklin. I have turned up three interesting drag reducing things:

(1) Minix Wing Tip designed by Christian Hugues that eliminates the wing tip vortex. http://www.minix.fr  It would make a sturdy wing tip not susceptible to wing rash. You can tell everyone the wing tip cylinders are for air to ground rockets! Claims -8% reduction in induce drag, lift increase 5.5%, increased L/D 2.4%and an increase in Aspect Ratio of 9%

(2) Gurney Flap that increases lift without any drag. NASA TM 4071 and "Numerical Investigation od an Airfoil with a Gurney Flap" Aircraft Design 1 (1998) 75-88. It analyses 4412 section. Same as on the Stinson. The Gurney Flap is 1.25% Chord 90 degrees at the trailing edge. It improves the circulation around the airfoil by creating a vortex that pulls down the airflow coming off the top of the wing.

(3) In order to balance the Stinson with the much heavier engine installation and to reduce the stall speed for my grass runway I need to extend the Stinson airfoil forward 13.5" increasing the chord to 72". I have John Dresse's 2D DesignFOIL program. This is where "A little Knowledge is a dangerous thing" comes in. I started trying all sorts of airfoils to see if I could find one that I could graft over the existing 4412. After a lot of internet research and trying all sorts of combinations I happened to see RAF 6 in his airfoil archive. I remember this from my model aircraft days and know it is used on propellers due to the flat bottom. I plotted it and it fits like a glove. In the program I extended the wing span from 33'-11" to 36' with a 72" chord. The total wing drag with the old RAF 6 was 58% that of 4412. Significant enough to catch my attention. The RAF 6 with the sharp 0.42%Radius leading edge seems to be the reason. The 4 digit modified airfoil section in DesignFOIL allow the parameters to be adjusted. A sharp leading edge, the maximum camber location moved forward and 10% sections almost duplicate the low drag results. The problem with the RAF6 is it has a lower Cl max and stalls at a lower angle of attack. The Stinson has efficient Fowler flaps so the lift is OK. It is the sharp stall that could be a problem. The solution is a full span automatic retractable Leading Edge Slat. My internet searches really haven't yield much hard data on the optimum shape of Leading Edge Slats. I did find NACA TN 1293 May 1947 "Tests of the NACA 64A212 Airfoil section with Slat, a Double Slotted Flap and Boundary Layer Control". Also I found "High Lift Aoerodyamics" by A. M. O. Smith (Douglas Aircraft) June 1975 that explains how the L. E. Slat and Slotted Flap really work.  By my poor, questionable estimates  the modified Stinson at 2400 lbs (pilot & fuel) would float along at 80 mph on 3.5 GPH. At 3000 lbs (pilot, three passengers and fuel) and 120 mph it would burn 5.7 GPH. The L/D is 20.6. The LS1 specific fuel consumption at low power settings should be 0.35 lbs/HP hr. Some data suggests as low as 0.28 lbs/HP hr. With the Franklin at 110 mph the fuel burn was 10+ gph. This potential for lower fuel consumption suits my type of local sight seeing flying from my own airstrip. Also the LS1 burns premium auto gas and can tolerate ethanol content due to the in tank pumps and a 60 psi fuel rail to the injectors.

    Does anybody have better information on Leading Edge Slats?

Regards,

Leigh Scott

 

 

 

 



John McGinnis
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#16 Posted: 11/26/2009 00:18:25 Modified: 11/27/2009 22:40:57

Leigh,

Thanks for the post. You're biting off far more death than I'd recommend, but perhaps others will be more lenient. I'm not an aggressive modifier of other people's designs. I do believe in liquid cooling and paying sharp attention to induced drag. Airfoils can be improved, but you'd have to have full command of the subject or someone willing to help. The weight change of your proposed solution is tough enough! Regarding your points, I'd like to comment in order.

 

1. No such thing as 'eliminating' wing tip vortex. It can only be weakened and/or reoriented. I presume you meant only to draw attention to the Minix technology for the former result, which I think does work based upon studying its operational principle. I cannot vouch for the numbers. I was happy to have seen it at an aerospace conference a few years ago and got to speak in pantomime aeroEnglish with its creator.

Minix is a tip mounted vortex tube with a helical slot. At the cost of some parasitic drag, it reorients a disruptive stream of airflow in a manner that interferes with vorticular tip flows, shortening the time scale for breakdown, thereby lowering the maximum vortex velocity and strength. It does not increase aspect ratio, but any induced drag reduction equates to an increase in aspect ratio, whether there is one or not. 9% is a possibly believable number; winglets and various forms of dihedral achieve more.
nouveau_minix_aero.jpg

This is, in fact, a statement about span efficiency. Prandtl gives a maximum of 47% for the boxplane, and several other non-planar treatments get close. Tall winglets get 20% or so.

Should you put them on your Stinson?

Depends on the paint job, I suppose. Olive drab? You're a brave man ;-)

 

2. The various devices called "Gurney flaps" encompass a broad range of ideas all credited to Dan Gurney and used extensively in the auto racing world. The sharp trailing edge cuff you describe is the real deal, an original innovation by accident, and it's not so much a flap as a strip of angle! Yes, it does work, particularly at low Reynolds numbers. However, the aerodynamic cost to a Stinson probably breaks the budget.

Adding lift enhancement at the trailing edge invariably leads to a change in pitch moment, which means a total destabilizing of the design configuration. Not only must the tail work harder to balance the rearward shift, at stall the aircraft will tend to pitch up as the center of lift reverts to its natural quarter chord location. As to its 'zero added drag' status, this is rarely if ever true, but when (sometimes) it is, it's because of fairly abysmal airfoil performance in the first place.

Some modern airfoil designs have a pronounced Gurney flap trailing edge on purpose. At the same conference, Swift Engineering's Killer B prototype had a trailing edge cusp about as aggressive as any I've seen. Personally, I think 'gently' is the way to treat all the air sliding past our airplanes, and things designed to spin a vortex should be small and serve an important purpose.

 

3. I have no problem recommending a total airfoil replacement. Major gains are possible. However, what you need isn't just another catalog foil. It needs to have the proper behaviors for the remaining attributes of the aircraft as configured, and that's not to be taken lightly. IF you had compatible max lift, stall break, thickness, and (most germane to your post) pitching moment, and appropriate structure for any change in max lift, you could expect great things. Personally, I'm not into quite that much plastic surgery on vintage females.

Harry Riblet gives some examples of this level of rework in his book GA Airfoils, available through EAA. You may find the principle of how it can be done gives food for thought.

Be sure to include slats in the conceptual design of your custom foils. They certainly can improve things. I don't have specifics for you though.

 



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Eric Peterson
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#17 Posted: 11/27/2009 05:54:43

Years ago I saw a picture of a wing tip that is very similar to the one in your pic. I think it was called a Finch wing tip. Eric



Joanne Palmer
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#18 Posted: 11/29/2009 09:32:45

John:

 

While this is an interesting exercise, the two examples you give as approaching the G-VK limit line are classic examples of purpose-built designs.  Burt Rutan knew that to get maximum range he had to create aircraft that had L/D ratios that approached gliders.  So he essentially motorized a pair of "Siamese Twinned" gliders.  The two boom designes were for fuel capacityand to get the required rigidity in the wings. 

 

There really aren't many secrets in aircraft design.  And if there were more efficient methods of aircraft shaping for aerodynamics, these would have been pursued by now.  If for no other reason, the fuel shortages of the 1970s and 1980s would have dictated that we do more with less fuel.  However the efficiencies found have not been in aerodynamics but rather in the parasitic systems like air conditioning, electricity generation and hydraulic power generation.  Those and higher and higher bypass ratios with their resulting increases in turbine pressure ratios.  In short the secret to fuel efficiency has been in propulsion.  This has been true for at least thirty years.   Modern airliners having a flat plate ratio of around 0.09 or less ( as does the lowly Cessna 172) there isn't that much that can be done economically.   At least not with the reliability needed for aircraft design.  The rule here is that undetectable flaws causing catastrophic failures have to be mathematically eliminated to a probability of about 1 in 100 million.  Electro-kinesthetic wing warping anyone? 

 

The same could be said for GA.  While the 1940s designs still in use today can use some significant drag improvement, where the efficiency gains are lie in propulsion.   Propeller design has improved some, but real improvements have yet to be implemented at the GA level.  We can do better with our engine cooling than we have been, pressure cowls and air cooling constitute about 30 percent of the drag on a GA single.  That's a lot of drag and most of it doesn't need to be there.  Liquid cooled geared high speed engines have been shown as the way to go for decades but we still fly air cooled direct drive engines.... 

 

But everyone thinks aerodynamics is the key....sigh....

 

 



Eric Peterson
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#19 Posted: 11/29/2009 17:54:46

Would the upcoming electric driven prop community(LSA) have someting to contribute? Light weight and higher endurance batteries. It appears that electric only aircraft would have a much smaller cowliing and very small cooling intakes. Continued development of batteries could someday soon make for higher,faster,and farther possible. Same goes for motors. Electraflyer has an interesting site and photo gallery.

Also continued evolution of carbon composites. I'm looking for a recent article about a new generarion of carbon clothes. Supposed to be stronger than current. They have something to do with nano technology.

Just some food for thought.   Eric



John McGinnis
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#20 Posted: 11/30/2009 01:37:21 Modified: 12/1/2009 00:32:40

Joanne,

As illustrated by the special purpose designs mentioned, aerodynamics ARE the key to approaching the Gabrielli - von Karman limit. Their low induced drag (by span increase in this case) is mission critical. There are easier ways to build a gas tank. Yet rather than concur with your statement that "everyone thinks aerodynamics is the key", your post provides a good example for my argument, which could easily be summarized as: "almost nobody thinks aerodynamics is the key."

One reason for this is that most people, including many who are paid to know such things, are unaware of the gulf between what is possible, and what we have achieved. We tend to think that aerodynamics has explored the full realm of what is possible.

No way. Not even close. And the idea that if there were any 'more efficient ideas out there for aircraft shaping' that we'd have applied them by now is something you probably know to be false. There are hundreds of factors opposing the adoption of great ideas into commercial application, and hundreds of such ideas. Most unused aerodynamic insights were sidelined by regulation, complexity, and cost, yet many still die of institutional apathy and ignorance.

In fairness, you've not been exposed to the designs I am talking about. I hope when you are, you'll recognize their right to compete with prior work.

Another reason is that the prevalent approach to aircraft design recognizes aerodynamics as a discipline within the process, and multiple disciplines exercise an influence. Sometimes we credit other disciplines for the decisions leading to a particular form. My position is that this perspective is fundamentally limiting.

The things you have mentioned are all a part of the aerodynamic question, and should be subject to it. But at the same time, we shouldn't consider airplanes and airplane parts as if these were somehow a study of aerodynamics.  To improve aerodynamics is to study viscous fluid flow and work in harmony with it. It's not about the airplane anymore. It's properly about 'what we are trying to do to the air, and why'. No discussion of the 'how' can be had until the fact that we drag the sky along with us when we fly is no longer debated.

Your statement that 'there really aren't that many secrets' anymore is true! I think there are only one or two major 'secrets' left, but they are huge. And they're not even secret! Anyone can read classic papers that make the case I am making. Soon they will be able to study new applications of these principles that resolve some of the issues that have long stood in our way, such as dependence upon long wingspan for induced drag reduction.

That's why this forum exists, and why it is entitled Beyond Streamlining.

"100% perfect streamlining" would STILL leave more than 50% (!) of the opportunity for 'drag reduction' on the table. Aerodynamics has concerned itself with its first 50% commendably in two regimes: over 450 MPH, and under 100 MPH. At the high end, with machines such as the 747 and Concorde, we actually cross the original GvK limit. But at the low end, streamlined though they are, our best efforts still don't get close.

In between those speeds, we barely know what we're talking about. Unless we accept this truth, and know why it is truth, we would argue its existence while standing on the evidence.

Even the engine efficiencies you point to are another example of the triumph of.... fluid dynamics! High bypass 'jet' engines are turbine powered ducted fans, no longer trying to blast air out the back in pure high speed reaction, but rather, to move more of it less and to suck more in the front. Propulsive efficiency is half of what powers the future, and it's about aerodynamics.

My view is that everything you mentioned (including the flux dynamics of efficient electromagnetic generation) IS fluid dynamics to those with the luxury to think that way. (I'll grant you that it's not the perspective found in the average global conglomerate.) Arguments in favor of liquid cooling, propulsive efficiency, and structural common sense flow from such a perspective far more readily than they do from the status quo.

The gaggle of planes we love dearly, at the bottom of the chart, clearly do not represent the pinnacle of achievement in aerodynamics. If aerodynamics is charged with the study of fluid motion around aircraft, these aircraft represent an era in which it was extremely difficult to reduce that study to a form of design practice. They flow from equations and assumptions that EXPRESSLY ignore many, many things in order to simplify the problem. Most were designed long before the hand held calculator, using slide rules and graph paper. The people who designed them were extremely well qualified to do so, but not well equipped to perform the kinds of analysis possible (often in seconds) today.

To suggest that a flat plate ratio of .09 is some kind of benchmark of success is to be in the dark about what we're discussing here. Someday we'll find that number pathetic. Indeed, to think in terms of flat plate area at all is to take the first of many detours into the hysteresis loop built into our system for killing the consideration of solutions in their conceptual infancy. We just can't get 'there' using that approach. Using what we know more comprehensively will reveal aircraft showing practical utility in every application, while at the same time reducing complexity and cost.

Finally, the stunning achievement of Rutan's special purpose designs is in the economy of structure and the audacity to build them in the first place. Aerodynamically, there is little new there beyond John Roncz airfoils. I take great comfort in the fact that Burt prevailed through sheer determination to demonstrate (repeatedly) that those who believe "there isn't that much that can be done economically" are frequently in error.


 



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